1. Field of the Invention
The present invention relates to the improvement of an optical module to be used in unidirectional or bidirectional communication.
2. Description of the Background Art
In the optical communication, an optical signal is produced by a semiconductor laser (LD) at one node, the optical signal is sent through an optical fiber, and the optical signal is read out by a photodiode (PD) at another node. Transmission and reception of the signal is performed by using two optical fibers in one case and one optical fiber in another case. As the optical source, a semiconductor laser (LD), a light-emitting diode (LED), or the like is used. Here, it is assumed that an LD is used as the light source.
When it is intended to send the largest possible number of signals by using a single optical fiber, the wavelength division multiplexing transmission is employed. Two or more signal lightwaves Λ1, Λ2, . . . having different wavelengths are produced by different LDs at different nodes. The signal lightwaves are combined to send over a single optical fiber. The transmitted signals are separated at the exit according to the wavelengths. The separated signals are detected by different PDs at different nodes.
As a multiplexer for combining lightwaves having different wavelengths and as a demultiplexer for separating lightwaves having different wavelengths, a WDM (in this specification, the term WDM is used to mean a wavelength multiplexer or demultiplexer) and the like are used. When a signal is sent in one direction, a plurality of transmitting nodes are provided at one end of the optical fiber. The transmitting nodes are denoted as U1, U2, U3, . . . Um. At the other end, the same number of receiving nodes are provided. They are denoted as W1, W2, W3, . . . Wm. The light sources for all the transmitting nodes are semiconductor lasers LD1, LD2, LD3, . . . LDm having different oscillation wavelengths. The different oscillation wavelengths are denoted as Λ1, Λ2, Λ3, . . . Λm. The demultiplexer can completely separate the lightwaves Λ1, Λ2, Λ3, . . . Λm having different wavelengths. Therefore, it is supposed that no interference occurs between different nodes.
FIG. 19 shows a simplified optical communication system. For simplicity, in this system, m=2 is employed. However, the same phenomenon occurs in a general system in which m=m. The LD1 at the U1 generates a transmission lightwave of ΛA. The lightwave propagates through an optical fiber 38, a multiplexer (WDM) 39, an optical fiber 40, a demultiplexer (WDM) 42, and an optical fiber 43 to enter a PD1 at the receiving node W1. In this case, however, from the U2, although the magnitude is small, another lightwave of ΛA may enter the PD1 at the W1, causing a noise.
Similarly, the receiving node W2 receives the signal lightwave ΛB from the U2, together with a small magnitude of the lightwave ΛB from the U1. This small magnitude causes a noise. In other words, the W2 receives the signal lightwave from the U2 and a noise form the U1. This example has only two pairs. However, in the case where the system has “m” pairs and they transmit and receive signal lightwaves through a common optical fiber 40, all the nodes may have a possibility of receiving not only a single signal lightwave but also “m−1” noise lightwaves.
In the case where a central office and each of the terminal optical network units (ONUs) are connected with a single optical fiber and the system performs bidirectional communication by sending an analog signal and a digital signal, there also exists a possibility of interference. FIG. 20 shows schematically such a wavelength division multiplexing optical communication system. A central office has a node Ua for transmitting and receiving a digital signal and a node Ub for transmitting an analog signal. The node Ua is provided with a LDa for converting a digital signal into an optical signal of ΛA and a PDa for converting an optical signal of Λc into a digital signal. A digital signal from the Ua is sent as an optical signal of ΛA and travels through an optical fiber 48, a multiplexer (WDM) 49, and an optical fiber 50 to arrive at a PDc at the terminal Wc. The PDc receives the signal. An LDe at the terminal ONU generates a digital signal having a wavelength of Λc. The digital signal is transmitted to the PDa at the Ua in the central office through the optical fiber 50, the multiplexer (WDM) 49, and the optical fiber 48.
If this is only the operation, it is simply a one-to-one bidirectional communication. However, in addition to the above operation, the LDb in the Ub at the central office transmits an analog signal ΛB having a wavelength of ΛB. The analog signal travels through an optical fiber 54, the multiplexer (WDM) 49, and the optical fiber 50 to enter a PDd at the Wc. In such a case, the two lasers LDa and LDb in the central office have different oscillation wavelengths of ΛA and ΛB. This causes interference in the PDc and PDd. Even though the multiplexer (WDM) 49 operates normally, the interference occurs.
The present inventor has studied variously about the cause. Finally, the present inventor has determined the cause of the interference. A semiconductor laser is supposed to generate a stimulated emission lightwave having a single wavelength of Λ. Actually, however, in addition to that emission, a semiconductor laser also generates a weak lightwave. According to the theory of the lasing, in the case where a population inversion is produced by a current, when a stimulated emission occurs, all the excited electrons are supposed to return to the ground state simultaneously, generating an oscillation at the same phase and the same wavelength. It is explained that when the excitation energy exceeds a threshold value, all the energy is absorbed into a mode having a common phase and wavelength, producing only a lightwave having a single wavelength of Λ.
The actual fact is different, however. At the time the electrons return to the ground state, a lightwave is emitted that has an energy corresponding to the level difference between the conduction band and the valence band. In this case, the electrons in the conduction band also have kinetic energy, and the holes in the valence band also have kinetic energy. Therefore, even when the electron-hole pairs disappear due to the returning of the electrons from the conduction band to the valence band, a small difference in energy exists from pair to pair. This energy difference may produce a plurality of longitudinal modes. Even when only a single longitudinal mode exists, a lightwave, although weak, is produced that has a wavelength other than Λ.
In the case of a semiconductor laser, which has a cavity short in length and has a large gain, a lightwave having a wavelength other than the oscillation wavelength Λ still exists. So far, the existence of such a lightwave has not even been noticed. The present inventor has found that such a lightwave having a wavelength other than Λ is emitted from a semiconductor laser. Here, the thus emitted lightwave is referred to as a “natural emission lightwave.”
The term “natural emission lightwave” is slightly different in meaning from the term “spontaneous emission lightwave” used in the laser theory. Originally, a laser is devised as a solid laser or a gas laser. In this case, the excitation level is distinct and the energy difference in the population-inverted level is determined uniquely. There exist no various energy levels. Therefore, the wavelength of the generated lightwave is determined as a single wavelength. Both the spontaneous emission lightwave and the laser oscillation lightwave have the same wavelength. When the spontaneous emission lightwaves become to have a uniform phase, they become to be the laser oscillation lightwaves. In other words, when the phase is uniform, they are the laser oscillation lightwaves. When the phase is randomly distributed, they are the spontaneous emission lightwaves. At least, both the spontaneous emission lightwave and the laser oscillation lightwave have the same wavelength (energy).
In the case of a semiconductor laser, however, the light emission is not produced by the two-level transition of atoms but by the current injection. Consequently, the light emission by the electron-hole pair produces different wavelengths. Here the term “natural emission lightwave” is used to have a wider meaning than the “spontaneous emission lightwave” used in the ordinary laser engineering. The term “natural emission lightwave” has a meaning different from the term “spontaneous emission lightwave” used in the ordinary laser engineering based on the explanation of the gas laser. Here, the emitted lightwaves having different energies (wavelengths), not different phases, are referred to as the “natural emission lightwave Σn.” To emphasize that the natural emission lightwave includes a large number of wavelengths, the sign Σ is used.
FIG. 21 is a wavelength spectrum of a semiconductor laser emitting a lightwave at a wavelength of Λq (for example, 1,490 nm). The horizontal axis represents the wavelength, and the vertical axis the power (dB). The laser oscillation lightwave is used as a reference of 0 dB. A peak of an output having a large power appears at an intended wavelength. However, in addition to that, a continuous spectrum having a weak output exists at both sides of the peak. This phenomenon has so far been overlooked. Not only is the phase different but also the wavelength (energy) is different. Here, the weak lightwave is referred to as the natural emission lightwave Σn. In contrast, the lightwave having a specified wavelength emitted by the laser is referred to as the laser oscillation lightwave Λq. The natural emission lightwave Σn is as weak as −40 to −50 dB. However, although weak, the natural emission lightwave exists.
Such a natural emission lightwave is likely to cause the interference in FIGS. 19 and 20. In the case of FIG. 19, the transmission member transmits both the signal lightwave ΛB from the LD2 and the noise lightwave ΛB from the LD1, and they enter the PD2 of the receiving member to be detected. The PD2 receives both the signal lightwave and the natural emission lightwave both having a wavelength of ΛB. The natural emission lightwave is a noise. In FIG. 19, the PD1 receives both the natural emission lightwave of the LD2 and the laser oscillation lightwave of the LD1. Both the PD1 and PD2 receive the noise having the same wavelength as that of the signal lightwave. Even when the multiplexer (WDM) 39 and the demultiplexer (WDM) 42 operate normally, because the wavelength is the same, such a noise cannot be blocked by the WDM. The present invention intends to solve the problem of the interference due to the natural emission lightwave.
So far, the fact has not been known that a semiconductor laser emits a natural emission lightwave having a different wavelength. Consequently, no literature has noticed the problem of interference that occurs when two or more semiconductor laser lightwaves having different oscillation wavelengths are transmitted through the same optical fiber. The present inventor was unable to find a previously published literature that raised such a problem and showed a step to solve it.
The patent literature 1 describes as follows: The oscillation wavelength of a semiconductor laser deviates from a specified value. To solve the problem, the backward light of the semiconductor laser is observed with a photodiode for monitoring the wavelength. When the wavelength deviates, a control circuit of the semiconductor laser returns the wavelength to the specified value. To prevent the backward light from leaking, the heat sink is extended toward the rear. This is an idea of the patent literature 1. The patent literature 1 notices as a problem the variation in the laser oscillation lightwave itself of a semiconductor laser. Then, the variation in the wavelength of the laser oscillation lightwave is prevented. The patent literature 1 does not mention the existence of a natural emission lightwave in addition to the lightwave having the oscillation wavelength Λ. It is likely that the patent literature 1 believes that the total energy is concentrated to the lightwave of the oscillation wavelength. In addition, the patent literature 1 does not raise the problem of interference caused by the natural emission lightwave.    Patent literature 1: the published Japanese patent application Tokukai 2003-229626
As described earlier, in a solid laser and the like, the spontaneous emission lightwaves are lightwaves having the same wavelength as that of the laser oscillation lightwave but having nonuniform phases. It is not recognized that in the case of a semiconductor laser, lightwaves having different wavelengths are emitted. In other words, it is not noticed that something like the natural emission lightwave exists that has a continuous spectrum with different wavelengths as described in the present invention. Consequently, no recognition exists to prevent the interference caused by that. In this field, it can be said that the natural emission lightwave itself in a semiconductor laser is a novel discovery. Therefore, no prior art exists to prevent the interference caused by that. The present inventor was unable to find such a prior art.